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Instruction Manual
IM-106-910Xi, Original Issue
November 2010
5-5
Xi Advanced Electronics
Probe Passes
Calibration, O
2
Still
Reads High
External Reference Air Leak - There may be a leak that is permitting
ambient air to mix with the process gases. Since many combustion processes
are slightly negative in pressure, ambient air can be drawn into the cell area,
biasing the O
2
reading upward.
1. Make sure that the calibration gas line is capped tightly between
calibrations.
2. If autocal is used, make sure the check valve is seating properly.
Figure 5-2. Probe Leakage
Paths
Internal Reference Air Leak - See Figure 5-2. There may be a leak inside
the O
2
Probe itself, permitting the reference air (20.95% O
2
) to mix with the
process gases at the cell. To confirm this leak condition, pressurize the inside
(reference side) of the probe by plugging the reference air exhaust port with
your finger for 1 minute. (The conduit ports where the signal and power wires
pass may also need to be sealed.) The O
2
reading should decrease slightly. If
the O
2
reading increases during this test there is a leak inside the probe.
1. Acid condensation inside the probe can degrade the hose that carries
the cal gas to the cell. Inspect this hose. Dislodging or improper
installation of the cal gas or reference air hose can cause a leakage
path.
2. The sensing cell is fastened to the end of the probe tube and uses a
corrugated washer to separate the process gases from the ambient
reference air. The corrugated washer may be damaged by corrosion.
Discard used washer.
NOTE
You should aways install a new corrugated washer whenever you remove the
sensing cell from the probe.
Bad Reference Side Cell Electrode - A bad reference side cell electrode can
cause an elevated O
2
reading.This fault is usually indicated by a frequent
"Calibration Recommended" alarm and increasing cell impedance readings. A
high cell impedance can be calibrated out, but if the impedance continues to
increase rapidly, the sensing cell must be replaced.